PLL-tuning system

ABSTRACT

A time discrete PLL-tuning system comprises a phase detector and a voltage controlled oscillator (VCO) for tuning the frequency (f VCO ) thereof to a frequency equal to N/M times a reference frequency (f REF ), with M a factor indicating the number of frequency steps in which a transmitter/receiver channel distance is divided and N the number of frequency steps in which the oscillator frequency is divided. The sampling frequency of the phase detector is substantial equal to the reference frequency (f REF ).

[0001] The present invention relates to a time discrete PLL-tuning system, comprising a phase detector and a voltage controlled oscillator (VCO) for tuning the frequency (f_(VCO)) thereof to a frequency equal to N/M times a reference frequency (f_(REF)), with M a factor indicating the number of frequency steps in which a transmitter/receiver channel distance is divided and N the number of frequency steps in which the oscillator frequency is divided.

[0002] Such a tuning system is generally known and is applied in radio's tv's, mobile telephones, etc. In the phase detector the difference between the phase of an input signal with a predetermined frequency step (f_(STEP)) and the phase of a signal derived from the VCO output signal by dividing the frequency thereof with the factor N is detected, filtered en fed back to the VCO. When the frequency step (f_(STEP)) is equal to a reference frequency (f_(REF)) divided by the factor M, the VCO frequency is tuned to N/M times said reference frequency, i.e. to the frequency (N/M)*f_(REF). The reference frequency may be generated by a cristaloscillator.

[0003] The accuracy of the frequency of the tuned oscillator on the one hand and the switching speed to switch the oscillator between different frequencies on the other hand are limited as a consequence of the low sampling rate of the control loop which locks the tuned oscillatorfrequency to a frequency proportional to the fixed reference frequency. The best choice of this sampling rate or sampling frequency is the frequency step fSTEP of the tuned oscillator, which frequency step is usually a fraction (1/M) of the channel distance in telecommunication systems, i.e. f_(STEP)=(1/M)*f_(REF). In that case, in practice, the bandwidth of such a the tuning system will be about f_(STEP/)10 (instead of the theoretical Niquist criterion f_(STEP)/2). For high accuracy the frequency step and therefore the sampling frequency would be low. This requirement results in a small bandwidth of the sampled control system. However, the small bandwidth, in turn, results in slow switching speeds between different channels. The switching time from one channel to another will be about 1/f_(STEP); the exact value depends on the step size and accuracy. Further, due to the sampling mechanism of the control loop, during tuning always a spurious component with a frequency equal to the frequency step (f_(STEP)=f_(REF)/M) will be present in the control loop. This means that a frequency modulated VCO signal will be obtained with the consequence that signals will be received from a transmitter transmitting in a neighbouring channel at a frequency distance f_(STEP) from the frequency of the VCO. To keep the spurious signal sufficiently low, heavy loop filtering has to be applied. However, such a loop filtering results in a small bandwidth of the control system and thus to a slow switching speed.

[0004] Further from an article of Amr N. Hafez an M. I. Elmasry, presented at the IEEE 1999 Custom Integrated Circuits Conference a wide band tuning system based on a dual (nested) FLL/PLL architecture is described with a PLL as described above. There, the FLL (frequency locked loop) comprising a frequency-to-voltage feed back loop, integrated in the PLL, has a large bandwidth, a high reference frequency and low accuracy, while the PLL has a small bandwidth, a low reference frequency and high accuracy. In this way the function of small step size/high accuracy and the function of of large loop bandwidth are separated. Due to this separation, however, a disadvantage with respect to fast switching from one channel to another is introduced because the exact frequency is controlled by the slow PLL only. While the VCO frequency is tuned to the frequency step (indicated as reference frequency in said article), the FLL provides for a suppression of the phase noise. The FLL presented in this paper is based on a frequency-to-voltage converter which is critical with respect to high frequency operation. In order to apply this circuit the clock frequency (spurious) of the FLL has to be reduced via a frequency divider compared to the high frequency of the VCO. That means that the fundamental advantage of the FLL can not be fully exploited.

[0005] The purpose of the invention is to provide for a time discrete PPL-tuning system wherein the disadvantages of the above tuning systems are avoided, and which tuning system is accurate and has a high switching speed to switch the oscillator between two different frequencies.

[0006] Therefore, according to the invention the time discrete PLL-tuning system is characterized in that the sampling frequency of the phase detector is substantial equal to the reference frequency (f_(REF)). In the generally known PLL-tuning systems the sampling frequency is, as mentioned above, equal to f_(STEP); according to the invention the sample frequency is M*f_(STEP). This means that the spurious component has a frequency M*f_(STEP) which can be chosen far beyond the range of present neighbouring transmitter/receiver channels, so that a wide band tuning system can be obtained and thus a rapid switching speed, while the frequency step can still be low, resulting in a high tuning accuracy. The bandwidth of the PLL-tuning system according to the invention is about f_(REF)/10, which is a factor f_(REF)/f_(STEP)=M wider than the bandwidth of the generally known PLL-tuning system.

[0007] Such a high sample frequency may be realized in a PLL-tuning system in which the phase detector comprises a first frequency-to-voltage converter, which in response to the VCO signal supplies an output signal (k′*f_(VCO)) proportional to the oscillator frequency (f_(VCO)), a multiplier unit which in response to said output signal (k″*f_(VCO)) of the frequency-to-voltage converter supplies a signal (k*M*f_(VCO)) proportional to the product (f_(VCO)) of the oscillator frequency and a factor M, indicating the number of frequency steps in which a transmitter/receiver channel distance is divided, a reference signal unit which in response to a reference frequency signal supplies a signal (k*N*f_(REF)) proportional to the product of the reference frequency (f_(REF)) and a factor N, N being the number of frequency steps in which de oscillator frequency is divided, and a difference circuit supplying a signal proportional to the difference of the product of the oscillator frequency (f_(VCO)) and the factor M and of the product of the reference frequency (F_(REF)) and the factor N, which difference signal (k*[M*f_(VCO)-N*f_(REF)]) is subjected to low filtering and as control signal is supplied to the VCO. This means that the VCO control signal V_(c) can be represented by:

V _(C) =K*M*∫[f _(VCO) −N/M f _(REF) ]*dt,

[0008] which relation will be the same as can be derived for the generally known PLL's mentioned before. The important difference is that in this embodiment in spite of a low frequence step the sample frequency is much higher than in the generally known systems, so that in this embodiment V_(SPURIOUS) is formed by a signal with frequency f_(REF), while in the known PLL's V_(SPURIOUS) is formed by a signal with frequency f_(REF)/M

[0009] The signals M and N may be supplied in digital form and are preferably adjustable. In such an embodiment the multiplier unit may comprise a first DAC (digital-to-analogue converter) with a current output to convert the signal M into a current, and a first current switch controlled by the output signal of the first frequency-to-voltage converter. Further, in order to obtain an substantially identical electronic structure, in such an embodiment the reference signal unit may then comprise a second frequency-to-voltage converter, and a second DAC (digital-to-analogue converter) with a current output to convert the signal N into a current to be supplied to a second current switch, which second current switch is controlled by the output signal of the second frequency-to-voltage converter.

[0010] In a preferred embodiment each of the first and second frequency-to-voltage converter comprises an exclusive-or circuit to which the signal with the oscillator frequency and with the reference frequency respectively are supplied directly and after a delay (τ), and low-pass filter means for filtering the output signal of the exclusive-or circuit. The delay (τ) may be adjustable, which is important to obtain an accurate substantially equal adjustment of both values of the delay (τ) in both frequency-to-voltage converters.

[0011] In the following and with reference to the accompanying drawing an embodiment according to the invention will be described more in detail. In the drawing:

[0012]FIG. 1 shows a generalized block schematic diagram of the PLL-tuning system according to the invention; and

[0013]FIG. 2 shows a preferred embodiment of the PLL-tuning system according to the invention.

[0014]FIG. 1 shows a phase detector 1 and a voltage controlled oscillator (VCO) 2. The output signal of the phase detector 1 is the control voltage V_(c) for the VCO, while the output signal with the frequency f_(VCO) forms a first input of the phase detector 1. A signal with the reference frequency f_(REF) forms a second input signal of the phase detector 1. The phase detector 1 comprises a frequency-to-voltage converter 3, a multiplier unit 4, a reference signal unit 5, a difference circuit 6 and a low-pass filter 7. The frequency-to-voltage converter 3 supplies, in response to the input signal from the VCO, a voltage V=k′*f_(VCO), with k″ an adjustable factor or a constant. In the multiplier 4 this signal is multiplied with a factor M, indicating the number of frequency steps f_(STEP) in which a transmitter/receiver channel distance is divided. The output signal of the multiplier 4 may be represented by k*M*f_(VCO), with k an adjustable factor or a constant, and supplied to the difference circuit 6. The reference signal unit 5 supplies, in response to a reference signal with frequency f_(REF), an output signal, which may be represented by k*N*f_(REF), with N the number of frequency steps in which the oscillator frequency is divided and k an adjustable factor or a constant, and supplied too to the difference circuit 6. The circuits 3, 4 and 5 may so be dimensioned that the signals M*f_(VCO) and N*f_(REF) have the same proportionality factor or constant. The values N and M are supplied in digital form. The output signal of the difference signal M*f_(VCO)−N*f_(REF) will be integrated in the filter 7, whereafter the control signal

V _(C) =K*M*∫[f _(VCO) −N/Mf _(REF) ]*dt

[0015] is obtained, with an adjustable factor or a constant. The sample frequency of the PLL tuning system is f_(REF)=M*f_(STEP), with f_(STEP) the frequency step. The bandwidth of the control loop is about f_(REF)10. The spurious component with frequency f_(REF) is far out of the range of the transmitter/receiver channels.

[0016] A preferred embodiment is shown in FIG. 2. The main structure of the block schematic diagram in this figure is the same as in FIG. 1.

[0017] The frequency-to-voltage converter 3 comprises an exclusive-or circuit annex low pass filter 8. The output signal of the VCO is supplied to the exclusive-or circuit annex low-pass filter 8 directly and after a relatively small delay I via a delay circuit 9. The low pass filtered output signal of this frequency-to-voltage converter 3 is now proportional to τ*f_(VCO).

[0018] The multiplier unit 4 comprises a DAC (digital-to-analogue converter) 10 with a current output and a current switch 11. By means of a reference current i_(REF) the value M is converted in an current, proportional to M*i_(REF) and supplied to the current switch 11. Under control of the output signal of the frequency-to-voltage converter 3 a current i₁ will pass the current switch 11, which current iis proportional to τ*i_(REF)*M*f_(VCO).

[0019] The reference signal unit 5 has the same structure as the combination of the frequency-to-voltage converter 3 with the multiplier unit 4 and comprises a frequency-to-voltage converter 12, a DAC 13 with a current output and a current switch 14. Therefore, the output current i₂ is proportional to τ*i_(REF)*N*f_(REF). When the same DAC's, the same current switches and the same frequency-to-voltage converters are used, the proportionality constants of i_(1 and i) ₂ are also the same, so that the output signal of the difference circuit will be proportional with

τi_(REF) *[M*f _(VCO) −N*f _(REF)].

[0020] After integration and with τ*i_(REF)=K, the same formula is obtained as given above.

[0021] In order to realize equal proportionality constants for i₁ en i₂ the not only the reference signal unit 5 has the same structure as the combination of the frequency-to-voltage converter 3 with the multiplier unit 4, but also the delay τ is adjustable.

[0022] There are many implementations of the delay τ depending on the required accuracy. The absolute accuracy of the delay, however, is not important because it only effects the loop gain of the control loop which is a rather insensitive parameter. The dominant aspect for accuracy of the VCO frequency is the relative matching of the two delays in the different branches of the phase detector 1 and the accuracy of the DAC's. The most accurate implementation of the delay is a digital implementation with D-flipflops. Here, again, there are several options for the common clock signal of the D-flipflops in the two branches. The only consequence on system level is that the frequency f_(CLOCK) of said clock signal must be higher than the highest input frequency, f_(VCO) or f_(REF). If this not the case there is always the possibility to add fixed dividers at the input of the two branches.

[0023] An interesting limiting case is the situation where f_(CLOCK) is equal to f_(REF). In this situation the product τ*f_(REF) becomes constant with the consequence that the output current i₂ of the branch with f_(REF) as input frequency becomes independent of the input frequency f_(REF), and will only be a function of the dc-current of the programmable DAC 13. Therefore, in this situation the phase detector 1 can be simplified by replaced by the frequency-to-voltage converter 3 and the multiplier 4, steered by the value M only.

[0024] Another interesting limiting case is the situation where f_(CLOCK) is equal to f_(VCO). In this situation the product τ*f_(VCO) becomes constant, with the consequence that the output current i₁ of the branch with f_(VCO) as input frequency becomes independent of the input frequency f_(VCO), and will only be a function of the dc-current of the programmable DAC 10. Therefore, in this situation the phase detector 1 can be simplified by the reference signal unit 5 steered by the value N only. 

1. Time discrete PLL-tuning system, comprising a phase detector and a voltage controlled oscillator (VCO) for tuning the frequency (f_(VCO)) thereof to a frequency equal to N/M times a reference frequency (f_(REF)), with M a factor indicating the number of frequency steps in which a transmitter/receiver channel distance is divided and N the number of frequency steps in which the oscillator frequency is divided, characterized in that the sampling frequency of the phase detector is substantial equal to the reference frequency (f_(REF)).
 2. Time discrete PLL-tuning system according to claim 1 , characterized in that the phase detector comprises a first frequency-to-voltage converter, which in response to the VCO signal supplies an output signal (k″*f_(VCO)) proportional to the oscillator frequency (f_(VCO)), a multiplier unit which in response to said output signal (k″*f_(VCO)) of the frequency-to-voltage converter supplies a signal (k*M*f_(VCO)) proportional to the product (f_(VCO)) of the oscillator frequency and a factor M, indicating the number of frequency steps in which a transmitter/receiver channel distance is divided, a reference signal unit which in response to a reference frequency signal supplies a signal (k*N*f_(REF)) proportional to the product of the reference frequency (f_(REF)) and a factor N, N being the number of frequency steps in which de oscillator frequency is divided, and a difference circuit supplying a signal proportional to the difference of the product of the oscillator frequency (f_(VCO)) and the factor M and of the product of the reference frequency (f_(REF)) and the factor N, which difference signal (K*[M*f_(VCO)−N*f_(REF)]) is subjected to low filtering and as control signal is supplied to the CVO.
 3. Time discrete PLL-tuning system according to claim 2 , characterized in that the signals M and N are supplied in digital form.
 4. Time discrete PLL-tuning system according to claim 3 , characterized in that the signals N and/or M are adjustable.
 5. Time discrete PLL-tuning system according to claim 3 or 4 , characterized in that the multiplier unit comprises a first DAC (digital-to-analogue converter) with a current output to convert the signal M into a current, and a first current switch controlled by the output signal of the first frequency-to-voltage converter.
 6. Time discrete PLL-tuning system according to any one of the claims 3-5, characterized in that the reference signal unit comprises a second frequency-to-voltage converter, and a second DAC (digital-to-analogue converter) with a current output to convert the signal N into a current to be supplied to a second current switch, which second current switch is controlled by the output signal of the second frequency-to-voltage converter.
 7. Time discrete PLL-tuning system according to claim 6 , characterized in that each of the first and second frequency-to-voltage converter comprises an exclusive-or circuit to which the signal with the oscillator frequency and with the reference frequency respectively are supplied directly and after a delay (τ), and low-pass filter means for filtering the output signal of the exclusive-or circuit. 